The release of the Fifth Assessment Report from the International Panel on Climate Change highlights the impacts which climate change is likely to have for many parts of the world. This comes after a number of recently published papers, which look at the effects of warming on the Greenland Ice Sheet. One recent study used airborne data on ice thickness from NASA’s Operation IceBridge to investigate how patterns of surface melting and ice discharge across Greenland have changed in recent years. A second study looked at recent changes in ice discharge from northeast Greenland, a region which until recently was thought to have been little affected by climate change.
A number of findings from these studies have been picked up by the media. As with previous studies of the Greenland Ice Sheet, key details are often sensationalized, with many reports suggesting that collapse of the ice sheet is imminent. But what is meant by collapse, and what would the implications be? By calculating the thickness of the ice scientists have worked out that should all the ice in Greenland melt, sea levels will rise between six and seven meters. So does that mean that coastal cities will be inundated within a few short years? Without sufficient background it is difficult to put recent trends into context.
So with that in mind let’s start by looking at what we know about the Greenland Ice Sheet. First of all, Greenland is big. At over two million square kilometers, it is roughly ten times the size of the United Kingdom. Approximately 82% of its surface is covered by the ice sheet, which has a volume of 2.38 million km3 and reaches a depth of 3,000 meters in places. However, to put its size into perspective, this volume is equivalent to only about 8% of the Antarctic Ice Sheet.
Greenland differs from Antarctica in another very important aspect. Antarctica has a number of mountain ranges buried underneath the ice. However in Greenland the mountainous areas are generally located at the coast and tend to ring the island. The center of Greenland is in effect a large bowl, occupied by the ice sheet. This topography means that ice from the interior can only drain to the ocean through a few large outlet glaciers, which occur wherever there are gaps in the coastal mountains. These glaciers are like major rivers, draining vast regions of the ice sheet.
The Role of the Outlet Glaciers
The largest outlet glacier in Greenland is Jakobshavn Isbræ, which drains an area of approximately 110,000 km2,or some 6% of the entire ice sheet. Like most of the major outlet glaciers for the Greenland Ice Sheet Jakobshavn Isbræ flows into a deepwater fiord. This massive glacier produces an estimated 35 billion metric tons of icebergs per year, around 10% of the Greenland total, and is believed to have been the source of the iceberg which sunk the Titanic.
The role of the outlet glaciers is therefore fundamental to any understanding of what is happening to the Greenland Ice Sheet. Prior to 2005, measurements suggested that approximately 58% of ice loss from the ice sheet occurred through ice discharge, rather than surface melt. Scientists also noted that the major outlet glaciers have been increasing in speed in recent years. For example, a recent study found that the maximum speed of Jakobshavn Isbræ, a few kilometres upstream from its terminus, had increased to around 17 km per year by the summer of 2013. This makes it one of the fastest-moving glaciers on Earth, and represents a fourfold increase in speed since the mid 1990s. This speed increase has been accompanied by thinning of the lower glacier and a by a retreat of the glacier terminus, which has moved back several kilometers in recent years. This occurs as icebergs break free of the terminus in a process known as calving. One of the largest calving events ever recorded was captured in this video.
It is likely that the local topography of the seabed has played an important part in the speed up of Jakobshavn Isbræ. Nonetheless similar speed increases have been observed for most of Greenland’s outlet glaciers. Scientists believe that the key to such speed increases may lie with the position of the grounding line, which is the point at which a glacier flowing into the ocean separates from its bed and begins to float. Beyond this point, there is no resistance to glacier flow so speeds increase rapidly.
Since most Greenland outlet glaciers terminate in deepwater fiords and are hundreds of meters thick, the base of the glacier comes directly into contact with water from the ocean depths. A number of recent studies have suggested that changes to ocean circulation patterns may be causing increased stratification of ocean waters at higher latitudes. The effect of this trend is that a layer of denser, saltier water is now entering the fiords in which outlet glaciers terminate and causing the glaciers to melt from beneath. This dense ocean water is trapped beneath a layer of meltwater from the glacier, which is less saline and has a comparatively low density. Increased melting at the glacier base causes the grounding line to retreat, lowering resistance to glacier flow, and thus causing the glacier to accelerate. Thus it appears likely that increased ice discharge from Greenland may be a direct consequence of ocean warming at mid and high latitudes, a process which illustrates a complex interaction between different elements of the Earth system.
Eventually it is likely that most outlet glaciers will retreat to the point that they terminate in shallow water, or on land. When they reach this point then it is likely that flow rates will drop dramatically, and a new equilibrium will be established. However it is estimated that it will take several decades for most of the major outlet glaciers to reach this stage. As long as they terminate in deep water, accelerated flow rates are likely to continue, and possibly even to increase, contributing significantly to sea level rise in the process.
Another mechanism which is believed to have contributed to increased flow rates of the main outlet glaciers is increased surface melting. During the summer, meltwater tends to collect in ponds on the surface of the ice. Weaknesses within the ice can often result in catastrophic drainage of these ponds. Elsewhere, rivers of meltwater flow across the ice sheet, before disappearing into the depths of the ice, through what are known as “moulins”. Much of this meltwater will eventually end up at the base of the ice sheet, where it provides a lubricating layer between the ice and the underlying ground, lessening resistance to glacial flow. While this effect is believed to be small compared with the effect of grounding-line retreat, it is nonetheless believed to be a contributor to the speed up of the outlet glaciers.
However, perhaps the most worrying conclusion from recent studies is that although ice discharge has increased considerably since the mid 1990s, it now only comprises a third of the total mass loss from the ice sheet; a near complete reversal from the situation prior to 2005, when ice discharge was the dominant process. Since 2009, some 84% of the increased mass loss has occurred through surface melting, marking a dramatic increase in the amount of melting now occurring. This trend was underscored over a four-day period in July 2012, when satellite measurements revealed that surface melting occurred over some 97% of the Greenland Ice Sheet. Such widespread melting has never been observed before in over 30 years of satellite observations, with positive temperatures even being reported at Summit Station, the highest point of the ice sheet.
Sea Level Rise
So clearly the Greenland Ice Sheet is losing mass much more rapidly than in the past, but is it in danger of collapsing? Recent studies suggest that if current rates of ice discharge continue, this alone is likely to raise global sea levels by around 3 cm by the end of the century. In a worst-case scenario, if the rate of ice discharge were to increase dramatically, it could cause sea levels to rise by as much as 8 cm over this time period. The big unknown is how much surface melting could occur over the same time. If rates remain similar to those of today, then sea level rise from Greenland alone is likely to be between 10 cm and 25 cm by the end of the century. However if surface melt rates increase, then this figure could be significantly higher.
So what would constitute a collapse? Even under high-end warming scenarios, it is estimated that it would take around 2000 years for the Greenland Ice Sheet to disappear. Often media reports talk about irreversible changes occurring, but as we have seen above, it is likely that within a few decades most major outlet glaciers will terminate in shallow water or on land. By this stage, they will probably have reached a new equilibrium, which will lead to a reduction in mass loss due to ice discharge. What cannot be accurately predicted are the effects of surface melting, which is dependent on future weather patterns. If there is an increase in the kind of melting events that occurred in 2012, then surface melting from the ice sheet as a whole has the potential to increase considerably. However whether this constitutes irreversible change is debatable, since future weather patterns are notoriously difficult to predict.
Greenland is currently undergoing rapid change, with both ice discharge rates and surface melting having shown major increases over the last few years. In combination with increased melting from Antarctica and from smaller ice sheets and glaciers worldwide, these changes are likely lead to significant rises in sea level by the end of the century. This has major implications as far as coastal cities and low-lying areas are concerned. However there is a tendency for the media to sensationalize what is happening to the Greenland Ice Sheet. What is currently occurring is simply a natural change in response to human-driven climatic change, which will play out over the long term. The Greenland Ice Sheet is not going to disappear any time soon. Terms such as irreversible collapse are generally not helpful and often serve to mask the true, but less dramatic, implications of climate change.
Enderlyn, E., Howat, I., Jeong, S., Noh, M., Van Angelen, J., van den Brooke, M., 2014, “An improved mass budget for the Greenland ice sheet”, Geophysical Research Letters, vol 41 (3), pp866-872, http://onlinelibrary.wiley.com/doi/10.1002/2013GL059010/abstract
Khan, S.,Kjær, K., Bevis, M., Bamber, J., Wahr, J., Kjeldsen, K.,Bjørk, A., Korsgaard, N., Stearns, L., van den Broeke, M., Liu, L., Larsen, N., Muresan, I., 2014, “Sustained mass loss of the northeast Greenland ice sheet triggered by regional warming”, Nature Climate Change, 4, pp292-299, http://www.nature.com/nclimate/journal/v4/n4/full/nclimate2161.html
Joughin, I., Smith, B., Shean, D., Floricioiu, D., 2014, “Brief Communication: Further summer speedup of Jakobshavn Isbræ”, The Cryosphere, 8, pp209–214, http://www.the-cryosphere.net/8/209/2014/tc-8-209-2014.html
National Snow and Ice Center: Quick Facts on Ice Sheets, http://nsidc.org/cryosphere/quickfacts/icesheets.html